This invention relates to digital and analog television and, more particularly, to the provision of an overlay signal in a television signal to mitigate multipath effects as the television signal is propagated through space from a transmitter to a receiver.
One of the most important prevalent problems associated with the transmission of television signals is the problem of multipath effects. The term multipath, as used herein, refers to the propagation of electromagnetic waves along various paths from the television transmitter to the television receiver. Multipath effects may arise from fixed structures, such as building walls, acting as reflectors in the transmission channel. Moving objects, such as airplanes, may also cause a multipath condition. Even microreflections in cabling can cause multipath conditions. These structures can cause transmission of the television signal to occur along more than one path from the transmitter to the receiver. As a result, the same television signal may be received more than once, and at different times by one or more television receivers. The result of multipath effects in analog television is to create "ghosts" in the displayed television image. In digital television, the effects of multipath include moderate to severe degradation in the displayed TV picture and sound.
Various methods and systems have been designed to address the problem of multipath. See, for example, P. T. Marhiopoulos and M. Sablatash, "Design of a Ghost Canceling Reference Signal for Television Systems in North America", Proceedings of Canadian Conference on Electrical and Computer Engineering, Vancouver, BC, Canada, Sep. 14-17, 1993, pp. 660-663.
The statistics of multipath ghosts have been studied and compiled by, among others, the BTA (Japan's Broadcasting Technology Association). The BTA, and other concerns, designed a "ghost canceling reference" (GCR) transmitted signal to mitigate these multipath induced effects. The BTA GCR was found to be less than satisfactory in some cases. While homes with outdoor antennas displayed non-varying (stationary) ghosting conditions which could be largely corrected, those homes with indoor antennas experienced changing (dynamic) ghosts. These ghosting conditions were more prevalent where people were moving about the room or other moving objects were in the signal path. The BTA ghost canceller generally was not able to adequately compensate for these dynamic conditions. Therefore, a need remains for a system and method for mitigating multipath effects in television systems, and especially for systems and methods for mitigating multipath effects caused by moving objects and dynamic conditions.